Method of recovering persistent photoconductors

ABSTRACT

A METHOD OF RECOVERING PERSISTENT PHOTOCONDUCTORS IS DISCLOSED. AFTER A FIRST IMAGE IS PRODUCED ON THE PHOTOCONDUCTIVE SURFACE, THE SURFACE IS UNIFORMLY CHARGED TO A PRE-SELECTED VOLTAGE TO PRODUCE A DEVELOPABLE IMAGE. THE FIRST CONDUCTIVE IMAGE IS NO LONGER DEVELOPABLE.

United States Patent 6 Int. Cl. Gil3g 13/22 US. Cl. 951 10 ClaimsABSTRACT UP THE DISCLOSURE A method of recovering persistentphotoconductors is disclosed. After a first image is produced on thephotoconductive surface, the surface is uniformly charged to apreselected voltage to produce a developable image. The first conductiveimage is no longer developable.

The invention relates to a photographic reproduction and, moreparticularly, to a method employing materials which exhibit persistentphotoconductivity.

Copending application Ser. No. 474,583, filed July 26, 1965, abandonedin favor of continuation-impart application, Ser. No. 847,493, filedJuly 14, 1969, describes a process wherein a persistentphotoconductivity member is exposed to a pattern of light to form alatent conductive image in the exposed areas. Next, the exposed surfaceis brought into contact or near contact with an insulating surface andthen separated or peeled away from the insulating surface while auniform external voltage is applied across the two surfaces. Anelectrostatic charge pattern corresponding to the conductive pattern ofthe photoconductor is formed on the insulating surface. This chargepattern on the insulating surface can be developed in any of theconventional ways.

While such a process is highly advantageous for making many copies fromone exposure, the persistent photoconductor must be recovered before thephotoconductive member can be exposed to a second different image. Thatis, recovery is defined to mean that the photoconductor can be used toprepare a developed copy of the second image without the first imagealso being visible on the developed copy.

A further requirement is that the recovery time must be short for themethod of Ser. No. 474,583 to be carried out in a reproduction devicecapable of producing single and multiple copies of the differentoriginals. Normally, recovery of the persistent photoconductor isbrought about by heating, which erases or removes the conductive image.The rate of recovery by heat erasure can be increased by increasing thetemperature. However, the necessity of increasing the temperature togain speed or a short recovery time is disadvantageous because manyphotoconductors show instability when heated, especially repeatedly andin conjunction with exposure to radiation. Thus, the lifetime of thepersistent photoconductors becomes limited.

To overcome the shortcomings of heating, other methods of recovery havebeen conceived. One of these methods has been to expose the persistentphotoconductor to infrared radiation, but this technique also appears towork through the generation of heat and has the disadvantages of heat.Moreover, it is less effective than the direct application of heat.Another recovery method is to use direct or alternating electric fields,but with this technique complete recovery of the conductive image isalmost never achieved so that the previous images are visible insubsequent copies.

Therefore, it is the primary object of this invention to provide a newand improved recovery method.

3,573fi39 Patented Mar. 30, 1971 ice Another object of this invention isto provide a recovery method in which recovery is complete and yet canbe achieved at a high rate of speed without shortening the lifetime ofthe photoconductor.

Still another object of this invention is to provide a recovery methodwhich can be used in conjunction with heat erasures without the heaterasure shortening the life time of the photoconductor.

In general, the foregoing and other objects and other advantages areachieved by the following method.

A first conductive image is present in a persistent photoconductor and,by the method of Ser. No. 474,583, one or more copies have been preparedby bringing the surface of the photoconductor into contact with aninsulating surface and then separating them, with a unidirectionalelectrical potential applied across the surfaces, to form, on theinsulating surface, an electrostatic image of the conductive image.

Next, to recover the persistent photoconductor in order to prepare acopy of a different image and in accordance with the present invention,the photoconductor is electrostatically charged with the potentialapplied in the same direction as the potential applied during the stepof forming the electrostatic image on the insulating surface and of amagnitude such that the resultant surface potential on the conductiveimage is greater than VV wherein V is the potential applied during thecharge formation on the insulating surface and V is the criticalpotential at which no charge formation occurs. That is, the polarity ofthe potential applied to the back of the photoconductor during recoveryis the same polarity as the potential applied to the back of thephotoconductor during charge formation.

Now, the photoconductor is exposed to a second and different image and,by using the method of Ser. No. 474,583, a copy of the second image isprepared without the first image being visible on the developed copy.HOW- ever, because the recovery process does not erase the conductiveimage, both the first and second images may be made visible on thephotoconductor by developing it with toner.

The mechanism of the recovery process of the present invention can beexplained by an understanding of the charge formation process of Ser.No. 474,583 which is simply defined by the equation:

wherein V is the potential on the insulating surface before chargeformation, V is the potential on the surface of the persistentphotoconductor before charge formation, V is the applied potential, andV is the resultant potential of V V and V and is proportional to thepotential across the gap between the photoconductive surface and insulartion surface. Charge formation will occur when V, is

greater than V the critical potential below which charge formation willnot occur. V is a function of the width of the gap between thephotoconductive surface and the insulating surface, and the gas presentin the gap.

Normally, V is assumed to be zero as is V assumed to be zero in theexposed areas of the photoconductor before charge formation. V is atleast 400 volts with the preferred voltage being about 700 volts.Therefore, with the condition that V and V in the exposed areas areassumed to be zero, V =V. In addition, the 400 volts lower limit of Vsets a practical limit on the potential required for charge formation tooccur with V and V at zero potential. That is, for this condition, V isestablished at slightly less than 400 volts for a minimum gap width ofless than 7n and with air as the gas in the gap. Herein, however, forpurposes of description, it will be referenced at 400 volts.

Now, after a persistent photoconductor has been exposed to one image andused in the charge formation process of Ser. No. 474,583 to preparecopies of that image, it must be recovered so that the photoconductorcan be exposed to a different image. In accordance with the presentinvention, the surface of the photoconductor is electrostaticallycharged with the potential applied to the back of the photoconductorhaving the same polarity of the potential applied to the back of thephotoconductor during the subsequent charge formation and of a magnitudesuch that the resultant surface potential, V in the exposed areas of thephotoconductor is greater than V 'Vfc- The potential V to be appliedduring the subsequent charge formation step normally will be the samemagnitude as the potential applied during the previous charge formationstep.

For purposes of illustrations, a 600 potential is applied to the back ofthe photoconductor during all charge formation steps and, for purposesof recovery, the surface of the photoconductor is electrostaticallycharged by a 7000 volt corona unit. That is, a -7000 volt potential isapplied to the back of the photoconductor, so that the polarity of boththe charge formation potential and the electrostatic charging potentialare the same. However, for ease in solving the equation, the back of thephotoconductor will be referenced at zero potential and, hence, thecharge formation potential or V is +600 and the electrostatic chargingpotential is +7000. Accordingly, from the electrostatic charging step orrecovery step of the present invention, the previously unexposed areashave a resultant surface potential of about +1000 volts and thepreviously exposed areas have a resultant surface potential of about+350 volts, depending on the particular photoconductive composition.Turning now to the equation:

wherein V can still be assumed to be zero, but now V in the exposedareas is +350 volts. With V at +600 volts, it can be seen that:

V =+250 volts Inasmuch as V is established 400 volts, in thisillustration +400 volts, V; is less than V and no charge formation Willoccur. This agrees with the requirement of V e V-V for recovery of theexposed areas of the photoconductor. That is, 350 volts is greater than200 volts (600 volts400 volts). Of course, with the unexposed areashaving a 1000 volt surface potential, charge formation will not occur inthose areas. Now, if the photoconductor is exposed to a different image,V in the newly exposed areas or conductive image again can be assumed tobe zero. Thus, in the new conductive areas, V V, and hence V will be+600 volts which is greater than V (+400 volts) and charge formationwill occur. Thus, developed copies of the different image can beprepared on insulating surfaces by the process of Ser. No. 474,5 83without the first conductive image also being developed.

Any photoconductor capable of forming a latent conductivity image onexposure can be used in the process of the present invention. Thisincludes photoconductors, such as those described in copendingapplication Ser. No. 474,977 filed July 26, 1965 now US. Pat. 3,512,966,which can be recovered by one of the above known methods and alsophotoconductors, such as those described in US. Pat. 3,113,022, whichcannot be recovered by one of the above known techniques, i.e.heat. Ofcourse, photoconductors, such as selenium, whose conductivity does notremain or persist after the exposing light is removed and, hence, arenot useful in the method of Ser. No. 474,583 or, in other words, theprocess of the present invention is not applicable to non-persistentphotoconductors.

Turning now to a discussion of the method of Ser. No. 474,583 in aconventional electrophotographic machine environment with thephotoconductor carried on a drum to show how the recovery method of thepresent invention improves the method of Ser. No. 474,583 so that it canbe employed as a high speed reproduction method. In the method of Ser.No. 474,583, a previously unexposed persistent photoconductor is exposedto a pattern of light and dark from a first original or master to form aconductive image in the photoconductor. Next, the exposed surface of thephotoconductor is brought into contact with the insulating surface of adielectric sheet and, at least during the separation of thephotoconductor from the dielectric sheet, a potential is applied acrossthem, which is suflicient in magnitude to form an electrostatic chargepattern on the insulating surface corresponding to the conductive image.Preferably, an electrostatic charge is applied to the surface of thephotoconductor prior to the application of the potential across thephotoconductor and dielectric sheet and of such a polarity to preventelectrostatic charge formation on the insulating surface of thedielectric sheet corresponding to the non-conductive or insulating areasof the photoconductor. This step is only necessary before the firstcharge formation copy because the charge will be retained on thenon-conductive or insulating areas of the photoconductor during theformation of subsequent charge patterns on dielectric sheets of the sameconductive image.

Of the above steps in the method of Ser. No. 474,583, it primarily isthe exposure step which limits the speed of operation in making one copyof the first original. This is because the charge formation step and thepreferred charging step can be carried out at a higher rate of speedthan the exposure step of the photoconductor due to the lightsensitivity of the photoconductor. Generally speaking, thephotoconductor can travel at a speed of about at least 200 linear feetper minute for the charge formation and charging steps whereas,depending on the particular photoconductor, can only travel at a speedof about 20 linear feet per minute if the exposure step is optical andabout linear feet per minute if contact exposure is used. However, onlyone exposure is needed for multiple copies of the same original so that,overall, the machine employing the method of Ser. No. 474,583 canoperate as a high speed reproduction device if only multiple copies ofthe same original is desired.

On the other hand, if the machine is to produce copies of differentoriginals by contact exposure using the method of Ser. No. 474,583, thespeed of operation is primarily dependent upon the heat recovery orerasure time of the photoconductor because the photoconductor can onlytravel at about a speed of about 60 linear feet per minute for C. heaterasure step. However, if in place of the heating step of Ser. No.474,583 the recovery method of the present invention is used, thelimiting step on the speed of operation again is the exposure step inexposing of the second original. That is, the recovery meth 0d of thepresent invention only requires an electrostatic charging step which canbe carried out at about at least 200 linear feet per minute, a fasterrate of speed than the exposure speed of the photoconductor.

Now, the photoconductor can be exposed to a pattern of light and darkfrom a second and different original and the charge formation method ofSer. No. 474,583 can be employed to make one or multiple copies of thesecond original. Again, no additional exposure step is necessary for themultiple copies. Therefore, it should be apparent that the presentinvention makes the method of Ser. No. 474,5 83 practical for use in areproduction device preparing not only multiple copies of one originalbut single and multiple copies of many different originals at a highrate of speed.

It will be remembered that the first conductive image was not erased bythe recovery method of the present invention because if thephotoconductor is developed by a conventional technique, such as cascadedevelopment, both the first and second conductive images are visible.These conductive images are not permanent, except for possibility thematerials of US. Pat. 3,113,022, and persist from minutes to daysdepending upon the particular composition of the photoconductor.Therefore, the con ductive images will gradually become erased at roomtemperature over a period of time. If desired, however, heat erasure canbe used in conjunction with the process of the present invention and canbe employed infrequently so as not to shorten the lifetime of thephotoconductor. For example, the heating means of Ser. No. 474,583 couldbe included in a high speed reproduction device and operated only whenthe reproduction device was being shut off, such as the end of the day.In this way, the conductive images would be erased without affecting thehigh speed operation of the reproduction device or shortening thelifetime of the photoconductor.

The general nature of the invention having been set forth, it now willbe further illustrated by reference to the following specific examplesin which the criticalness of the polarity of the potential applied tothe photoconductor for recovery of the conductive image will be shown.In addition, the following examples will show that the recovery processof the present invention is operable on both reversible and irreversiblepersistent photoconductive materials.

In these examples, the terms negative or positive, when they relate toelectrostatic charge patterns or toner, connote the polarity of thecharge of the pattern or the toner. On the other hand, when these termsare applied to a master, they are definitive as to whether theinformation or background is dark. That is, this patent is a positivemaster because the print, not the background, is dark. On the otherhand, if the master is a transparency in which the print areas aretransparent and the background dark, this would be a negative master.This same definition is true when the terms positive and negative areapplied to copies. A different master is a master that would interfereand overlap the image areas of a previous master. Of course, thephotoconductor can be re-exposed to the same master. In this case, therecovery step is employed so that accurate alignment of the twoexposures and, hence, conductive images is not necessary to prevent thecopies from being blurred.

EXAMPLE I A persistent photoconductive film was prepared by dissolving10 mg. of Malachite Green oxalate and 40 mg. 3,5-dinitrobenzoic acid ina small amount of methanol to which was added 14.3 g. of a solution of5.3% of polyvinylcarbazole in dichloroethane. This solution was coatedon an aluminum foil using a mil wet gap. The prepared film was exposedto a 40 watt incandescent lamp at 12 inches through a negative masterfor two seconds. The film then was run several times through a chargeformation station, each time being sandwiched with a dielectric paperhaving an insulating surface. The charge formation station wasmaintained at 600 volts with the positive polarity being applied to theback of the photoconductor. Positive electrostatic charge impressionscorresponding to the exposed areas of the photoconductive film wereformed on each dielectric paper and were made visible with cornmercially available positive going (negative) toner particles appliedwith a magnetic brush to form positive copies of the negative master.

To recover the photoconductive film for a different image, thephotoconductive film was electrostatically charged with a corona unit ata 7000 volt potential. That is, a positive 7000 volts was applied to theback of the photoconductive film and, thus, was the same polarity as wasapplied to the photoconductor back during the charge formation step.Next, the photoconductive film was exposed to a 40 watt incandescentlamp at 12 inches through a different negative master for two seconds.

After carrying out the same charge formation step and development step,the dielectric paper was observed to see if the prior conductive imagehad been formed and developed. The prior image from the first negativemaster was not visible. The above process was repeated several times,each time exposing the photoconductive film to a different image. Ineach case, the prior image(s) did not appear on the developed dielectricpaper, but only a good quality positive copy of the negative image, fromthe exposure after the electrostatic charging step or recovery step, wasproduced.

EXAMPLE II Using the photoconductive film of Example I and theprocedural steps of recovery, exposure of a first positive master,charge formation, recovery, exposure of a second and different positivemaster, and charge formation, positive copies were prepared of thesecond positive master. The polarity of the potential applied to theback of the photoconductor during both the charge formation step and theelectrostatic charging step for recovery was reversed from that ofExample I so that negative polarity was applied to the back of thephotoconductive film. The electrostatic charge impressions on thedielectric papers were developed by cascade development with acommercially available positive going (negative) toner. Good qualitypositive copies of the second positive master were obtained without theimages from the first positive master being visible.

EXAMPLE III Using the photoconductive film and procedure steps ofExample I, the photoconductive film was exposed to 20 different imagesand from one to twenty electrostatic impressions were formed ondielectric papers from each image. In each case, the previous image(s)was not visibly present on the subsequent dielectric copy. That is, onlya good quality copy(ies) of the image after recovery was developed.

EXAMPLE IV In support of the mathematical explanation previously given,the photoconductive film of Example I was exposed uniformly to a 40 wattincandescent lamp at 12 inches for two seconds. Using the chargeformation technique of Example I, six electrostatic charge impressionswere made on six dielectric papers using a 600 volt potential of thesame polarity as Example I. Next, the completely exposed photoconductivefilm was measured with an electrometer and found to have a surfacepotential of +20 volts. The same procedure then was carried out with anunexposed film which, when measured with the electrometer, had a surfacepotential of volts after six charge formations. However, when thesefilms were electrostatically charged with a corona unit at 6000 voltspotential and in the same direction as the charge formation potential,the surface potential of the exposed film was +350 volts and the surfacepotential of the unexposed film was +1000 volts.

EXAMPLE V Using the film formulation of Example I, a previouslyunexposed photoconductive film was exposed through a target to a 40 wattincandescent lamp at 12 inches for two seconds. According to therecovery process of the present invention, the film then was chargedwith a corona unit set at 6500 volts with the positive polarity appliedto the back of the photoconductor. Next, instead of carrying out acharge formation step with a dielectric paper, the film was given ashort uniform exposure to dissipate the electrostatic charge and, afterthis, the charge formation step was performed and a positiveelectrostatic charge pattern of the exposure image formed on adielectric paper which was developed. Thus, this example shows that,without a surface potential greater than V-V in the previously exposedareas being present, a charge formation image will still result. Also,the example shows that the electrostatic charging step for recovery doesnot erase a conductive image.

EXAMPLE VI For further understanding of the recovery process of thepresent invention, a previously unexposed photoconductive film of theformulation of Example I was exposed through a positive master to a 40watt incandescent lamp at 12 inches for two seconds. The photoconductorthen was charged by a corona unit at 6500 volts with the positivepolarity applied to the back of the photoconductor. Next, using thecharge formation technique of Example I, except that the polarity of thepotential was reversed and the negative polarity supplied to the back ofthe photoconductor, the photoconductor in contact with a dielectricsheet was passed through the charge formation station. That is, thepolarity applied to the back of the photoconductor by the corona unitand the charge formation unit was opposite rather than the same, whichis required by the recovery process of the present invention. With thisreverse polarity, an electrostatic image was formed and developed on thedielectric paper whereas if the polarity had been the same as in ExampleI, no image would have formed.

EXAMPLE VII To show that the above results occur with a negative masteras well as a positive master, the procedure of Example VI was followed,except that a negative master was used and the polarity of the potentialof the corona unit applied to the back of the photoconductor wasnegative and the polarity of the charge formation potential applied tothe back of the photoconductor was positive. Again, the exposed imagewas not recovered and an electrostatic image was formed and developed onthe dielectric paper.

EXAMPLE VIII A photoconductive coating was prepared according to U.S.Pat. 3,113,022, in which the conductive images are essentiallyirreversible or not erasable by known techniques. That is, a diazoniurnsalt, 0.02 gram of 2,5-dimethoxy 4 benzoylaminobenzene diazoniumfluoboride, was dissolved in 2 ml. of acetonitrile and the solutionadded to 10 percent solution of polymethylmethacrylate in 2-butanone.The resulting solution was coated on aluminum foil with a mil wet gap.The coating then was cured at 60 C. Using the procedural steps ofExample I, the coating was exposed through a negative master to a 500watt photoflood lamp at 12 inches for sixty seconds. Next, the film wascontacted to a dielectric paper and passed through a pair of conductiverollers at a potential of 600 volts and with the positive polarity beingapplied to the back of the photoconductor. The charge formation step wasrepeated three times to give three electrostatic charge impressions onthree dielectric papers, which were developed and yield good qualitycopies of the master. To recover the photoconductive film, the film thenwas charged with a corona unit at 7000 volts with the positive polaritybeing applied to the back of the photoconductor. Now, the photoconductorwas exposed to a second and different master and, by repeating thecharge formation step above, an electrostatic charge impressioncorresponding to the image of the second master was formed on thedielectric paper. After developing the electrostatic charge impression,the paper was examined and a good quality copy of the second image waspresent without the first exposure image being visible. Two more copiesof different masters were prepared using this recovery process. In eachcopy, the previous image was not visible.

EXAMPLE IX After the three different copies from three differentexposure images were prepared by the charge formation technique inExample VIII, the photoconductor was again electrostatically charged tothe same potential and the same polarity and the photoconductor wascascade developed with a commercially available negative going(positive) toner. All three different images to which the film wasexposed in Example VIII were developed and visible.

EXAMPLE X A photoconductive composition of a solution of 30 mg. of2,2,4,4,6,6'-hexanitrodiphenylamine, 5 mg. of Malachite Green oxalateand 0.5 g. of 1,3-diphenyl-5-(4-dimethylaminophenyl)-pyrazoline in 5 ml.of 1,2-dichloroethane was added to a solution of 2 g. of polystyrene in18 g. of benzene. The resulting solution was coated on an aluminum foilwith a 7 mil wet gap. The film then was exposed through a negativemaster to a 40 watt incandescent lamp at 12 inches for two seconds.Next, the film was passed through conductive rollers at 900 voltspotential to show that the recovery process of the present invention isoperable with electrostatic charging means other than a corona unit. Thepositive roller or polarity was applied to the conducitve aluminumbacking of the film. Next, the film was re-exposed to a second anddifferent master and several charge formations made with dielectricpapers at 600 volts potential, with the negative polarity being appliedto the back of the photoconductor as was the case in the previousrecovery step. The dielectric papers were developed in the same manneras Example I. In each case, the previous image was not visible on thedeveloped dielectric paper.

EXAMPLE XI To show that the recovery process of the present in ventionis operable with inorganic persistent photoconductors, a solution of 7.5g. of polymethylmethacrylate in 40 g. of toluene was added 0.006 g. ofRhodamine B in 3.5 g. of methanol and 22.5 g. of electronic grade zincoxide. The mixture was dispersed by a high speed stirrer and coated onan aluminum foil with a 3 mil wet gap. The thus prepared photoconductivefilm was exposed through a first negative master for 10 seconds to a 40watt incandescent lamp at 12 inches. Next, the photoconductor wascontacted with a dielectric paper and passed through a pair ofconductive rollers at a potential of 600 volts and with the positivepolarity being applied to the back of the photoconductor. Thee1ectrostatic charge pattern which formed on the dielectric paper wasdeveloped to yield a good quality copy. To recover the photoconductor, a6000 volt potential from a corona unit was applied to thephotoconductor, with the positive polarity being at the back of thephotoconductor. The recovered photoconductor then was exposed to asecond and different negative master with the same lamp, distance, andtime as described above. Repeating the charge formation and developmentprocedure used for the copy of the first master, a good quality copy ofthe image of the second master was produced without the first imagebeing visible. Finally, a good quality copy of a third and differentmaster was prepared without the first and second images being visible bythe above described steps of electrostatically charging thephotoconductor, exposing it through the third master, forming a chargeimpression on a dielectric paper and developing the charge impression.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that variations in form may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

1. In the process of forming a developable electrostatic charge patternon a member having an insulating surface by applying a unidirectionalelectric field across said member and the surface of a persistent oressentially irreversible photocondutive element contiguous with thesurface of said member and having a first conductive image, the step ofrecovering the photoconductive element comprising:

electrostatically charging the photoconductor while it still containssaid conductive image with the applied potential being in the samedirection as the potential applied during the step of forming theelectrostatic charge pattern on the insulating surface and of amagnitude such that the resultant surface potential on the conductiveimage is greater than V-V wherein V is the potential of said electricfield applied during the formation of the electrostatic charge patternon the insulating surface and V is the critical potential at which nocharge formation will occur, whereby a second conductive image can beinduced in the photoconductor and a developable electrostatic chargepattern corresponding to said second conductive image can be formed onan insulating surface without a developable electrostatic charge patterncorresponding to the first conductive image also being formed.

2. The process of claim 1 wherein said second conductive image isdifferent from said first conductive image.

3. The process of claim 1 wherein V is about 400 volts.

4. The process of claim 3 wherein V ranges from above about 400 volts toabout 900 volts.

5. The process of claim 1 wherein the conductive image formed in saidphotoconductor is essentially irreversible.

6. A reproduction process for producing copies of at least two originalscomprising the steps of:

(a) exposing a photoconductive member capable of exhibiting persistentor essentially irreversible photoconductivity to a pattern of light anddark from a first original to induce electrical conductivity in thelight exposed areas;

(b) placing a member having an insulating surface in contact with theexposed surface of the photoconductive member;

(c) separating said members with a uniform electric field applied acrossthe surfaces whereby an electrostatic charge pattern is formed on theinsulating surface corresponding to said pattern of light and dark;

((1) developing the electrostatic charge pattern to render it visible;

(e) electrostatically charging the photoconductive member while itcontains a conductive image with the applied potential being in the samedirection as the potential applied during the step of forming theelectrostatic charge pattern on the insulating surface and of amagnitude such that the resultant surface potential on the conductiveimage is greater than VV wherein V is the potential of said electricfield applied during the formation of the charge pattern on theinsulating surface and V is the critical potential at which no chargeformation will occur;

(f) exposing said photoconductive member to a pat tern of light and darkfrom a second original to induce electrical conductivity in the lightexposed areas; and

(g) repeating the steps b, c, and d whereby an electrostatic chargepattern of only the patern of light and dark from the second original isformed and developed on an insulating surface.

7. The process of claim 6 wherein said second original is different fromsaid first original.

8. The process of claim 6 wherein V is about 400 volts.

9. The process of claim 7 wherein V ranges from above about 400 volts toabout 9'00 volts.

lit). The process of claim 6 wherein the conductive image formed in saidphotoconductor is essentially irreversible.

References Cited UNITED STATES PATENTS 2,833,648 5/1958 Walkup 96--12,857,290 10/1958 Bolton 11717.5 2,982,647 5/19'61 Carlson et a1. 9613,005,707 10/1961 Kallman et a1 961 3,013,878 12/1961 Dessauer 9613,041,167 6/1962 Blakney et a1. 96-1 3,084,061 4/1963 Hall 11717.53,147,679 9/1964 Schatfert --1.7 3,268,331 8/1966 Harper 96-1 3,322,5385/1967 Redington 961.1 3,355,289 11/1967 Hall et al 96-1.4

GEORGE F. LESMES, Primary Examiner I. C. COOPER III, Assistant ExaminerUS. Cl. X.R.

961.3, 1.4; ll7--l7.5

